Butadiene-based aqueous dispersions, especially styrene/butadiene and acrylonitrile/butadiene latices, are widely used in various applications, such as paper processing agents and in the manufacture of rubber, for example tires. It is well known that the emulsion polymerization of butadiene or its copolymerization with other monomers produces a crosslinked polymer whose degree of branching increases as the degree of conversion increases. This crosslinking has a harmful effect on the working properties of the latices obtained. For example, it may have an effect on the coalescence of the latex particles during coating applications, for instance paper coating, or may contribute towards poor control of the hardness of rubber, thus limiting its processability.
To limit the degree of crosslinking and/or the molecular mass of the copolymer formed, it is possible either to work at limited conversion or to use a chain-transfer agent during the polymerization. The transfer agents commonly used are mercaptans, which, although having a very low residual content in the final latices, they induce an undesirable characteristic odor.
The aim of the present invention is to overcome the drawbacks of the prior art by proposing an efficient polymerization process, resulting in a good conversion of the monomers, which gives copolymer latices with a low degree of crosslinking. This low crosslinking may be exploited to reduce the mercaptan content and thus to reduce the odor or to increase the degree of conversion without obtaining prohibitive degrees of crosslinking.
According to the present invention, the process for obtaining aqueous dispersions of copolymers, which consists in polymerizing a blend of monomers containing at least 60% by weight of butadiene and at least one monomer chosen from styrene, acrylic esters, vinyl nitrites with optionally up to 40% by weight of one or more other copolymerizable unsaturated monomers, as an aqueous emulsion in the presence of one or more free-radical initiator(s), is characterized in that at least one of the initiators or the initiator is an azocarboxylic acid ester of formula (I):
in which:R1, R2, R3 and R4, which may be identical or different, are independently selected from the group consisting of                linear or branched alkyls containing from 1 to 9 carbon atoms and preferably from 1 to 4 carbon atoms, optionally substituted with one or more substituents selected from hydroxyl, C1 to C6 alkoxy and halogen substituents;        C3 to C12 cycloalkyls, optionally substituted with one or more substituents selected from C, to C6 alkyl, C1 to C6 alkoxy, hydroxyl and halogenated groups;        aralkyls optionally substituted with one or more C, to C6 alkyl, C1 to C6 alkoxy, hydroxyl and halogenated groups;        aryls optionally substituted with one or more substituents selected from C1 to C6 alkyl, C1 to C6 alkoxy, hydroxyl and halogenated groups;with at least one of the combinations R1–R2 and R3—R4 possibly forming an aliphatic ring; R″ and R′ are identical or different and are independently selected from the group consisting of linear or branched C1 to C10 and preferably C1 to C4 aliphatic radicals.        